The present invention relates to memory devices. In particular, the present invention relates to an efficient word line, bit line and precharge tracking in self-timed memory devices.
Referring to FIG. 1, a circuit diagram of a conventional memory device 100 is shown. The memory device 100 includes an internal clock line 102, a memory array 104, a first precharge transistor 106, a first transmission gate 108, an address decoder (XDEC) 110, a reference array 112, a reference column 114, a reference input/output (IO) 116, a reset clock 118, a sense amplifier 120, and a precharge line 122. All the internal clock signals in the memory device 100 are generated on the internal clock line 102. The memory array 104 includes a plurality of bit line columns 124, one of which is shown. The bit line column 124 includes a plurality of bit lines 126 (again, one of which is shown). The bit line 126 is connected to a plurality of memory cells, which are further connected to a plurality of word lines (not shown). The reference column 114 includes a Reference Bit Line (RBL) 128. The reference array 112 includes a dummy decoder 130 and a fifty (50) percent loopback 132. The reference IO 116 includes a second precharge transistor 134 and a second transmission gate 136.
The bit line 126 and the RBL 128 are precharged to a first predefined threshold voltage by a precharge signal (PRCH) that is generated on the precharge line 122 before a read operation. Precharging of the bit line 126 and the RBL 128 results in a high read current and a fast read operation. The first predefined threshold voltage is defined by a user, based on the voltage (VDD) applied to the memory device 100. The first predefined threshold voltage generally lies in the range of about 90 to 98 percent of the applied voltage VDD. Precharging of the bit line 126 and the RBL 128 is performed by the first and second precharge transistors 106 and 134, respectively. The drain terminals of both of the precharge transistors 106 and 124 are connected to the applied voltage (VDD) line. A word line clock signal is generated on a Word Line Clock Line (WLCLK) to start the read operation. The word line clock signal is provided to the XDEC 110, which generates a Word Line Enable Right (WLR) signal 138a and a Word Line Enable Left (WLL) signal 138b, to enable at least one of the plurality of word lines. The word line clock signal is also provided to the dummy decoder 130, which generates a reference word line signal in the reference array 112. The reference word line signal is generated at the same time as the XDEC 110 generates the WLR 138a and the WLL 138b, to provide simultaneous vertical tracking of the word line by the word line clock signal and horizontal word line tracking in the reference array 112. Horizontal word line tracking is performed by fifty (50) percent loopback 132, which includes dummy cells.
The reference word line signal is provided to the reference column 114. When the reference word line signal reaches the reference column 114, discharging of the bit line 126 and the RBL 128 are initiated. The reference column 114 includes dummy memory cells connected to the RBL 128. Therefore, the RBL 128 tracks the bit line 126 through the reference signal in the reference column 114. When the bit line 126 and the RBL 128 discharge to a second predefined threshold voltage, the first transmission gate 108 selects the bit line 126, and the second transmission gate 136 selects the RBL 128. The second predefined threshold voltage is defined as the voltage at which one or more inverters that are included in the sense amplifier 120 trip.
Thereafter, the reference IO 116 generates the reset clock 118, a Sense Amplifier Enable Right (SAER) signal 140a and a Sense Amplifier Enable Left (SAEL) signal 140b. The reset clock 118 pulls down the internal clocks generated on the internal clock line 102. The SAER signal 140a and the SAEL signal 140b enable the sense amplifier 120 to read the data stored on the bit line 126. The output of the bit line 126 is referred to as Q.
The memory device 100 described above suffers from wastage of area in the reference array 112 in spite of the advances made in memory architecture technology that have led to the development and use of 45 nanometer (nm) technologies. As explained with reference to FIG. 1, the reference array 112 includes the fifty (50) percent loopback circuit 132 for horizontal word line tracking. The remaining area of the reference array 112 is wasted, which leads to high fabrication cost for the memory device 100.
Further, the memory device 100 only uses logic delays to perform read and write operations. Logic delays are provided by the internal clock line 102 and generated in the memory device 100. Logic delays do not, however, provide sufficient accuracy to track variations in Resistance (R) and Capacitance (C) of the plurality of bit lines and the plurality of word lines, especially the levels of accuracy needed to comply with six sigma variations. Further, logic delays are not efficient in controlling the precharge signal generated on the precharge line 122. Therefore, tracking of the plurality of bit lines, plurality of word lines, and precharge are not accurate.